1. Field of the Invention
This invention relates to information recording apparatus using magnetic discs, rigid or flexible, for computer or for television purposes, and its aim is to maximize the information capacity of every disc surface. In particular, it relates to special methods and apparatus for increasing substantially the total number of recorded tracks, while at the same time equalizing their output signal properties.
2. Description of the Prior Art
Substantial effort and resources have been invested in magnetic disc recorders, particularly for computer applications. These machines are produced in very large yearly quantities and they have become an indispensable complement of every computer, mainly because they possess the ability to access rapidly any disc track. Their earlier rival, two or three decades ago, the magnetic drum, has practically disappeared, in spite of the fact that the drum excels in one important aspect over the disc: In the former all magnetic tracks have the same length and the same properties but in the disc the track length varies proportionally to its radius and the other track properties vary even more drastically. As a result, for the great majority of magnetic disc recorders whose discs rotate at a constant angular velocity and process information at a constant rate, the following disadvantages exist: First, only the innermost track is utilized fully at the maximum density of magnetic transitions, .delta..sub.i ; the density .delta..sub.j of the other tracks diminishes more and more as their radius increases and this deficiency decreases the disc utilization by a factor of 1.5, for the usual case where the ratio of the innermost track radius over the outermost, r.sub.i /r.sub.o, is approximately 0.5. Second, the head voltage produced in the playback mode and its frequency content varies as the radius, while its Signal to Noise Ratio (SNR) varies as the square. Obviously if all tracks could have uniform utilization and performance the quality of the machine would be enhanced and also its capacity would be increased. If, for instance, all tracks were to have the same SNR, the total capacity would increase by a factor of 2.33 for the same case as before, that is r.sub.i /r.sub.o =0.5, as will be shown in the description of the preferred embodiments section. In the past, machines have been made to correct the track density deficiency by dividing the disc surface into several zones, each processed at a different angular velocity, slower toward the disc's edge. This solution, however, is not acceptable at present because it forces the computer associated with this recorder type to change its information processing rate according to the zone. Another more practical approach is that of slanting the head gap variably as, for example, disclosed in the U.S. Pat. No. 3,384,880 of Duinker. But it is directed toward rectilinearly moving heads, while for reasons of cost and simplicity rotationally moving heads are more desirable. In addition, Duinker's method, which equalizes for constant output by increasingly slanting more heavily toward the disc's edge and thus continuously exchanging for effects due to increased relative velocity exactly opposing effects due to increased gap slant angle .psi.(.psi.=90.degree.-.phi., .phi.=angle between gap and track tangent), does not provide for phase coherence, the subject covered immediately below.
This invention eliminates the polarization difficulty of magnetic recording in the direction of the head's gap, meaning that to read any track recorded previously the gap must be both oriented in the same direction and stay within the track confines, for a wide range of cases. The first of these requirements is known in the industry as the Azimuth Alignment (AA) and the second as the Track Registration (TR). The two densities defining the total information stored in every disc surface, track density known as Tracks Per Inch (TPI) and circumferential binary digit density known as Bits Per Inch (BPI) and their product which characterises the storage efficiency in terms of imprints per square inch, are not compatible with both AA and TR with existing techniques. For example, for satisfactory Azimuth Alignment either TPI must increase and tracks must be narrow, or BPI must decrease and magnetic imprints must be placed far enough from each other. Also, to make the Track Registration easier TPI must decrease and tracks must be wide, but this will make the Azimuth Alignment more difficult, hence requiring a reduction of BPI.
This incompatibility becomes especially troublesome when considering the latest type of magnetic disc recorders using a servomechanism to correct for the various disturbances in achieving proper track registration by means of prerecorded and non-erasable special information known as "imbedded", in certain preassigned locations in every track known as "address and servo" sectors, because in order to read it correctly good registration must already exist and yet the object of the imbedded servo information is precisely to achieve good registration. Thus, the magnetic polarization and the incompatibilities it generates make the magnetic recording for computers inferior to the optical, for example, in total storage per square inch because its highest radial density at present (1985) is approximately 1,500 TPI while the optical discs have already exceeded 20,000 TPI.
Now, in the U.S. Pat. Nos. 3,701,846 4,175,270 4,322,759 and 4,388,655 of Zenzefilis, apparatus and methods are shown whereby the recording is changed into a new trackless type by means of recording all tracks coherently and contiguously and thus permitting video information to be read at all times without any track registration requirements and even when the head is moving from one location to another. Such machines have already been constructed and are able to process radial densities of more than 10,000 elemental tracks per inch. Coherence is achieved by taking all of the following measures:
(A) By breaking the incoming information stream into sections, and the sections into elements which are recorded on consecutive tracks in precise juxtaposition, the nature of the elements being such as to change smoothly from track to track.
(B) By making all tracks at least contiguous, and in some cases overlapped.
(C) By pulse length encoding the information content of each element using fixed angular disc positions such as a real or derived clock track as reference.
(D) By aligning corresponding elemental magnetic transitions from track to track so that they merge by means of aligning the head gap to be tangent on its path.
(E) By preassigning the two magnetization states between elemental transitions from track to track so they are the same.
Thus, all tracks disappear and all imprints merge forming long elemental lines, which are boundaries between the two states of magnetization of the disc's surface, either smooth if the corresponding elemental information is not changing from track to track or containing minute wiggles if there is change. All elemental lines have the same shape as the effective transducer's progress path (TPP). Although this transformation is specifically meant for TV signals in the above patents, it is also applicable to the address/servo sectors since their content varies smoothly from track to track. But the combination of variable slant and coherence requires TPPs that cannot be implemented by a simple mechanism.
It is the object of the herein described invention to expand and improve both Duinker's and Zenefilis' patents in order to achieve the following objectives either separately or simultaneously.
The Duinker variable slant is expanded to permit implementation by simple rotational head excursion movements as well.
The Zenzefilis' transducer progress path is expanded and improved by means of variably delaying the magnetic disc's reference position or positions or clock prior to recording the information meant to acquire phase coherence and be trackless by a delay function .tau.=g(r), so that the effective TPP on the disc is considerably changed from the apparent TPP. The effective TPP is neither a straight line for rectilinearly moving heads, nor a circular segment for rotationally moving heads, but a curve resembling an involute having at each radial distance the tangential inclination, .phi., needed to meet the particular requirement of equalizing the head output and also variably narrow the recorded tracks in order to accomodate more tracks.
Another object of this invention is to provide encoding means to record information which changes smoothly or predictably from track to track, such as the track serial numbers, or their radial distance, or a sector's serial number, etc., coherently and tracklessly, in order to transform the address sectors into areas of transverse elemental lines all having the general shape of the effective TPP. The actual computer information is still in the form of discrete tracks which are placed, as is usual, in between the address sectors. These tracks however, change progressively in width, becoming thinner toward the disc's edge, in exact correspondence to the elemental tracks within the address sectors, by virtue of the fact that the heads have the same inclination angle .phi. as the elemental lines either automatically so if the address sectors are prerecorded using the machine itself as a servowriter, or by construction so if the magnetic discs are prerecorded elsewhere by another machine.
3. Applicable Documents.
The statement earlier that the head output SNR varies as the square of the disc's radius can be supported by an article entitled, "On Extremely High Density Digital Recording", published in the IEEE Transactions on Magnetics, Vol. Mag-10, NO. 2, June 1974, page 368, by John C. Mallinson.